In the fabrication of a conventional LED, there is a difficulty in p-type doping of gallium nitride (GaN) material, which leads to a low concentration of hole carriers and limitations to the thickness of the material and thus forms a barrier for current diffusion. In order to obtain uniform current distribution, a current spreading layer is typically formed over the p-type GaN material.
However, use of the current spreading layer is also associated with some issues. On the one hand, the current spreading layer partially absorbs light and causes a reduction in light extraction efficiency. On the other hand, thinning the current spreading layer will make it less effective to form uniform and reliable current spread over the p-type GaN material. Therefore, its use requires a proper compromise between the light extraction efficiency and the current spread effect. This will inevitably impose limitations on further improvement of its power conversion efficiency. In addition, in the conventional LED, as electrodes and leads are arranged on the same side of a light-generating region, light is partially blocked away during operation and the efficiency of the conventional LED is thus limited. Further, the conventional LED relies on a sapphire substrate with a very low thermal conductivity for dissipation of heat from its p-n junction. This will lead to a very long heat conduction path in case of a power LED device having a large size. As a result, the conventional LED will have high thermal impedance, as well as a limited operating current.
In order to overcome the above-described drawbacks of the conventional LED, Lumileds invented a flip-chip LED in 1998. The flip-chip LED is fabricated by: preparing an LED die; preparing a submount sized correspondingly to the die and forming, on the submount, a conductive layer and conductive structures (gold solder balls for ultrasonic bonding) for connection of electrodes; and welding together the LED die and the submount. This structure additionally incorporates a metal reflector layer between its p-n junction and p-electrode, which directs light to exit from the sapphire substrate while not being blocked by the electrodes and leads. In addition, since light does not transmit through the current spreading layer, the current spreading layer is allowed to be made thicker so as to achieve a more uniform distribution of current density in the flip-chip LED. Further, this structure allows the conductive layer or metal solder balls to directly conduct heat from the p-n junction to the submount with a terminal conductivity that is 3-5 times higher than the conductivity of the sapphire substrate, thus resulting in a better heat-sinking effect. Therefore, this structure is more advantageous in terms of electrical, optical and thermal performance.
An existing method for forming the metal reflector layer is to electroplate a highly reflective metal onto the surface of the p-type GaN material. As a result, since light generated from the active layer is reflected at the bottom of the flip-chip LED, light is less absorbed at the bottom of the LED and the LED's light extraction efficiency can thus be improved. However, this architecture makes part of the light confined in a wave guiding structure formed by the layers of the LED. After several reflections therein, the part of the light will be attenuated or absorbed rather than escaping. This is detrimental to the light extraction efficiency.